1. Field of the Invention.
This invention relates to the simultaneous molding of polycarbonate lens blanks and the like, particularly the simultaneous molding of a plurality of lens blanks of either the same or different optical power. Although polycarbonate lenses are mentioned throughout this specification, it is understood that this invention is equally adaptable to molding other thermoplastic resins.
2. Technical Problems and Prior Act.
In the past, ophthalmic lenses and instrument lenses and small windows have been molded from polycarbonate resin and other thermoplastic materials. In order to be commercially acceptable, lenses must be free of blemishes and distortions and have edge portions free from starring that results from non-uniform setting within a mold cavity of the thermoplastic material of which the lens is composed.
The optical properties of molded lenses are difficult to control. Many parameters must be controlled to insure adequate optical and other physical properties of the polycarbonate lenses. These parameters include the quantity and temperature of the resin, mold temperature, clamp pressure, injection speed and pressure at which the molten resin is injected to fill the mold cavity, the pressure that the molding members apply to the resin entrapped in the filled cavity and the duration of the cycle steps. These factors must be correlated for each different size, thickness and optical power of mold blank produced.
It is imperative that polycarbonate lenses be produced having a minimum of stress. One factor that minimizes stress is the uniformity of application of molding pressure in the direction of the thickness of the blank throughout the extent of the major surfaces of the blank, at least during the time it takes for the resin to set. This result occurs when the charge of liquid polycarbonate fills the cavity completely so that the opposite molds engage both major surfaces of the molten charge throughout their entire extent throughout substantially the entire duration of the compression step that follows the step of injecting the molten resin into the mold cavity to form a finished article. Each different lens mass requires a unique minimum duration of substantially uniform compression to reduce stress to a value sufficiently low to insure adquate optical quality.
Stress is most difficult to avoid around the edge of the lens blank. Starring is present in lenses that are stressed along their edge portions. When an optometrist edges or machines a lens blank to conform to the shape of an eyeglass frame, starring due to grains caused by stress when the plastic is compressed non-uniformly within the mold cavity results in edge weakness that leads to eventual breakage or distortion of the lens. Immediate application of elevated pressure to compress the charge of molten polycarbonate must be uniform throughout the extent of the major surfaces of the charge.
When a plurality of lenses of equal size and optical power are produced simultaneously with prior art apparatus, it is most convenient for each cavity at each molding station of multiple mold station molding apparatus to be identical in all dimensions with those of the other stations. Otherwise, a combination of criteria suitable to produce a lens of a given size and optical power that is sufficiently free of stress to avoid excessive defects in said lens would not necessarily produce a lens of different size and optical power also free of defects.
Injection molding, compression molding and a combination of injection molding and compression molding have been used previously to fabricate polycarbonate lenses. Also, the prior art has used multiple molds that simultaneously produce identical molded parts from each of the molds of multiple mold apparatus. Unfortunately, a combination of parameters suitable to produce lenses of one size and thickness is not necessarily suitable to produce lenses of different sizes and thicknesses. Consequently, lens molding apparatus of the prior art, whether a single mold type or a multiple mold type, was capable of making lenses of a single size and shape and required a change of mold periodically. Production problems would be simplified if multiple mold apparatus were available to produce acceptable lens blanks of different size and optical power simultaneously.
U.S. Pat. No. 4,091,057 to Weber discloses a typical method and apparatus for injection molding polycarbonate lenses. The preferred embodiment of this invention takes approximately 10 seconds to inject molten plastic into a cavity and then applies 20 tons force for approximately 30 seconds. Only one molding station is present in this apparatus and a replaceable insert 52 or 57 is provided behind each molding member 59 or 60 to adjust the compression applied to the molten plastic in response to a change in thickness of a mold cavity within which the polycarbonate is injected. This apparatus cannot produce lenses of different sizes and optical power simultaneously.
U.S. Pat. No. 4,184,835 to Talbot discloses mold apparatus that combines both injection molding and compression molding principles to mold parts from thermoplastic resins. A cam 19 is actuated to move horizontally within a cam slot 20 by a piston 22 associated with a hydraulic cylinder 21 to lift a male mold member 13 towards a female mold member 11 during the last 20 to 25% of a mold cycle, preferably 5 to 10% of the last portion of the mold cycle, to provide the compression molding step of the molding cycle.
U.S. Pat. No. 4,309,379 to Dannels et al. discloses method and apparatus for compression and injection molding two or more articles of thermosetting molding material simultaneously. This patent does not suggest how one can simultaneously produce articles of different size and thickness in the different molds.
The enumerated patents lack a teaching of how to equalize the compression of the molten plastic regardless of differences in thickness of the mold cavity at different molding stations. The prior art could produce lens blanks of different thickness and different optical power simultaneously only when a different molding apparatus was used to develop a molding cycle most suitable for each product having a different thickness and optical power from others.